Aliphatic oligocarbonate diols are important precursors for the production of plastics, lacquers and adhesives, for example. They are reacted with isocyanates, epoxides, (cyclic) esters, acids or acid anhydrides, for example. They can be produced from aliphatic diols by the reaction thereof with phosgene (e.g. DE-A 1 595 446), esters of bis-chlorocarbonic acid (e.g. DE-A 0 857 948), diaryl carbonates (e.g. DE-A 1 915 908), cyclic carbonates (e.g. DE-A 2 523 352: ethylene carbonate) or dialkyl carbonates (e.g. DE-A 2 555 805 ).
Of the carbonate sources, diphenyl carbonate (DPC), which is a diaryl carbonate, is particularly important, since aliphatic oligocarbonate diols of particularly high quality can be produced from DPC (e.g. U.S. Pat. No. 3 544 524, EP-A 0 292 772). In contrast to all other carbonate sources, DPC reacts quantitatively with aliphatic OH functions, so that after removing the phenol which is formed, all the terminal OH groups of the oligocarbonate diol are available for reaction, e.g. with isocyanate groups. Moreover, only very low concentrations of a soluble catalyst are required, so that the latter can remain in the product.
Processes based on DPC have the following disadvantages, however:
Only about 13% by weight of the DPC remains as CO groups in the oligocarbonate; the remainder is distilled off as phenol. A significantly higher proportion of dialkyl carbonates remains in the oligocarbonate, depending on the alkyl radical concerned. Thus about 31% by weight of dimethyl carbonate (DMC) is available as CO for the oligocarbonate, since the methanol which is distilled off has a molecular weight which is considerably lower than that of phenol.
Because high-boiling phenol (normal boiling point: 182° C.) has to be separated from the reaction mixture, it is only diols with a boiling point considerably higher than 182° C. which can be used in the reaction, in order to prevent unwanted removal of the diol by distillation.
Due to their ease of production, dialkyl carbonates, particularly dimethyl carbonate (DMC), are distinguished as starting materials by being more readily available. For example, DMC can be obtained by direct synthesis from MeOH and CO (e.g. EP-A 0 534 454, DE-A 19 510 909).
Numerous publications (e.g. U.S. Pat. Nos. 2,210,817, 2,787,632, EP-A 364 052) relate to the reaction of dialkyl carbonates with aliphatic diols:
In the prior art, aliphatic diols are placed in a vessel together with a catalyst and the dialkyl carbonate (e.g. diethyl carbonate, dibutyl carbonate, diallyl carbonate), and the resulting alcohol (e.g. ethanol, butanol, allyl alcohol) is distilled off from the reaction vessel via a column. In the column, the higher boiling, dialkyl carbonate is separated from the lower boiling alcohol and is recycled to the reaction mixture.
Despite its ready availability, the use of dimethyl carbonate (DMC) for the production of aliphatic oligocarbonate diols has only recently become known (e.g. U.S. Pat. No. 5,171,830, EP-A 798 327, EP-A 0 798 328, DE-A 198 29 593).
EP-A 0 798 328 describes the reaction of the corresponding diol component with DMC with distillation of the azeotrope under normal pressure. Uncapping is subsequently effected by vacuum distillation, wherein degrees of utilization of the terminal OH groups of about 98% are achieved under very drastic vacuum conditions (1 torr, about 1.3 mbar) (EP-A 0 798 328: Table 1).
EP-A 0 798 327 describes a corresponding two-step process in which a diol is first reacted with an excess of DMC, with distillation of the azeotrope under normal pressure, to form an oligocarbonate, the terminal OH groups of which are present as methoxycarbonyl terminal groups and are completely inaccessible. After removing the catalyst and distillation of the excess DMC under vacuum (65 torr, 86 mbar) the oligocarbonate diol is obtained in a second step by the addition of further amounts of the diol and of a solvent (e.g. toluene) as an entraining agent for the methanol formed. The remainder of the solvent then has to be distilled off under vacuum (50 torr, 67 mbar). The disadvantages of this process are the cost of conducting it by the use of a solvent, and the repeated distillation which is required, as well as the very high consumption of DMC.
DE-A 198 29 593 teaches the reaction of a diol with DMC, with the methanol formed being distilled off under normal pressure. Apart from a single mention of the word “azeotrope” in the table headed “Process diagram of the process according to the invention”, no consideration is given there to the overall problem of the azeotrope. It can be calculated from the examples that DMC is used in excess and is azeotropically distilled off. About 27.8% by weight of the DMC used is lost.
According to U.S. Pat. No. 5,171,830, a diol is first heated with DMC and volatile constituents are then (azeotropically) distilled off. After vacuum distillation under drastic conditions (1 torr, 1.3 mbar), take-up of the product in chloroform, precipitation of the product with methanol and drying the product, an oligocarbonate diol is obtained in a yield of 55% by weight theoretical (loc. cit., Example 6). The degree of utilization of the terminal OH groups and the azeotrope problems are not considered in detail. Although U.S. Pat. No. 5,171,830 mentions, in column 5, lines 24 to 26, that the process can be conducted under vacuum, at normal pressure and at elevated pressures, and therefore can be conducted under all pressures, the particular preferences regarding the conditions of pressure employed cannot be identified. It is only a procedure which employs reduced pressure for the removal of volatile constituents which is mentioned.
Therefore, in the above publications, which were known hitherto, there is no description of a process, which is simple to carry out industrially, for the reaction of DMC with aliphatic diols to form oligocarbonate diols with high space-time yields and with high degrees of utilization of the terminal OH groups.
It is an object of the present invention to provide a simple, productive process, which can also be carried out on a large industrial scale, and which enables oligocarbonate diols to be produced by the transesterification of aliphatic diols with dimethyl carbonate, optionally with the use of an amount of catalyst which is low enough for the latter to remain in the product after completion of the reaction, with good space-time yields and with a high degree of uncapping of the terminal OH groups, in simple apparatuses.
It has now been found that the production of aliphatic oligocarbonate diols by the reaction of aliphatic diols with dimethyl carbonate, with the reaction optionally being accelerated by catalysts, at elevated pressure, results in a high space-time yield. In order to complete the reaction and in order to uncap the terminal OH groups (render the latter utilizable), the final residues of methanol and traces of dimethyl carbonate are removed from the product under reduced pressure, optionally with the introduction of inert gas.